The electrical activity generated in certain organs in the human body is intimately related to their function. Abnormalities in cardiac and brain electrical conduction processes are principal causes of morbidity and mortality in the developed world. Appropriate treatment of disorders arising from such abnormalities frequently requires a determination of their location. Such localization of the site of origin of an abnormal electrical excitation is typically achieved by painstaking mapping of the electrical activity on the inner surface of the heart or the brain from electrodes or a catheter. Often, this recording must be done while the abnormal biological electrical excitation is ongoing.
Radio frequency catheter ablation procedures have evolved in recent years to become an established treatment for patients with a variety of supraventricular [Lee, 1991; Langberg, 1993] and ventricular arrhythmias [Stevenson, 1997; Stevenson, 1998]. However, in contrast to supraventricular tachycardia ablation, which is highly successful because the atrio-ventricular node anatomy is known, ventricular tachycardia ablation remains difficult because the site of origin of the arrhythmia could be anywhere in the ventricles.
Sustained ventricular tachycardia is often a difficult arrhythmia to manage. One of the most common indications for radio frequency catheter ablation of ventricular tachycardia is arrhythmia refractory to drug therapy that results in frequent discharges from an implantable cardioverter-defibrillator. Radio frequency ablation is also indicated when the ventricular tachycardia (VT) is too slow to be detected by the implantable cardioverter-defibrillator or is incessant [Strickberger, 1997].
Selection of the appropriate target sites for ablation is usually based on a combination of anatomical and electrical criteria. The ability of the physician to deliver radio frequency energy through a catheter at the reentry site is restricted by the limitations of the current technology that is employed to guide the catheter to the appropriate ablation site. The principal limitation of the radio frequency ablation technique is the determination of the correct site for delivery of the radio frequency energy. Conventionally, this determination is achieved by painstaking mapping of the electrical activity on the inner surface of the heart from electrodes on the catheter. Often, this recording must be done while the arrhythmia is ongoing. This is a major problem, especially for those arrhythmias which compromise hemodynamic function of the patient. Many arrhythmias for this reason are not presently amenable to radio frequency ablation treatment.
The acute lesion created by radio frequency current consists of a central zone of coagulation necrosis surrounded by a zone of hemorrhage and inflammation. Arrhythmias may recur if the target tissue is in the border zone of a lesion instead of in the central area of necrosis. If the inflammation resolves without residual necrosis, arrhythmias may recur several days to several weeks after an apparently successful ablation [Langberg, 1992]. Conversely, an arrhythmia site of origin that was not initially successfully ablated may later become permanently nonfunctional if it lies within the border zone of a lesion and if microvascular injury and inflammation within this zone result in progressive necrosis [Nath, 1994]. Thus, the efficacy and long term outcome of catheter ablation depend on accurate determination of the site of origin of the arrhythmia.
Catheter ablation of sustained monomorphic ventricular tachycardia late after myocardial infarction has been challenging. These arrhythmias arise from reentry circuits that can be large and complex, with broad paths and narrow isthmuses, and that may traverse subendocardial, intramural, and epicardial regions of the myocardium [deBakker, 1991; Kaltenbrunner, 1991].
Mapping and ablation are further complicated by the frequent presence of multiple reentry circuits, giving rise to several morphologically different VTs [Wilbur, 1987; Waspel, 1985]. In some cases, different reentry circuits form in the same abnormal region. In other cases, reentry circuits form at disparate sites in the infarct area. The presence of multiple morphologies of inducible or spontaneous VT has been associated with antiarrhythmic drug inefficacy [Mitrani, 1993] and failure of surgical ablation [Miller, 1984].
Several investigators have reported series of studies of patients selected for having one predominant morphology of VT (xe2x80x9cclinical VTxe2x80x9d) who were treated with radio frequency catheter ablation [Morady, 1993; Kim, 1994]. It is likely that this group of patients represents less than 10% of the total population of patients with VT [Kim, 1994]. The patient must remain hemodynamically stable while the arrhythmia is induced and maintained during mapping. The mapping procedure may take many hours during which the arrhythmia must be maintained. Thus, currently radio frequency catheter ablation is generally limited to xe2x80x9cslowxe2x80x9d ventricular tachycardia (xcx9c130 bpm) which is most likely to be hemodynamically stable.
Ablation directed towards the xe2x80x9cclinical tachycardiaxe2x80x9d that did not target other inducible VTs successfully abolished the xe2x80x9cclinical VTxe2x80x9d in 71% to 76% cases. However, during followup up to 31% of those patients with successful ablation of the xe2x80x9cclinical VTxe2x80x9d had arrhythmic recurrences, some of which were due to different VT morphologies from that initially targeted for ablation.
Furthermore, there are several difficulties in selecting a dominant, xe2x80x9cclinical VTxe2x80x9d for ablation. Often it is not possible to determine which VT is in fact the one that has occurred spontaneously. In most cases, only a limited recording of one or a few ECG leads may be available. In patients with implantable defibrillators VT is typically terminated by the device before an ECG is obtained. Even if one VT is identified as predominant, other VTs that are inducible may subsequently occur spontaneously. An alternative approach is not to consider the number of VT morphologies in determining eligibility for catheter ablation but rather to attempt ablation of all inducible VTs that are sufficiently tolerated to allow mapping [Stevenson, 1998b; Stevenson, 1997]. However, this approach requires that the patient be hemodynamically stable during the VT mapping procedure.
The use of fluoroscopy (digital bi-plane x-ray) for the guidance of the ablation catheter for the delivery of the curative radio frequency energy is common to clinical catheter ablation strategies. However, the use of fluoroscopy for these purposes may be problematic for the following reasons: (1) It may not be possible to accurately associate intracardiac electrograms with their precise location within the heart; (2) The endocardial surface is not visible using fluoroscopy, and the target sites can only be approximated by their relationship with nearby structures such as ribs and blood vessels as well as the position of other catheters; (3) Due to the limitations of two-dimensional fluoroscopy, navigation is frequently inexact, time consuming, and requires multiple views to estimate the three-dimensional location of the catheter; (4) It may not be possible to accurately return the catheter precisely to a previously mapped site; (5) It is desirable to minimize exposure of the patient and medical personnel to radiation; and (6) Most importantly, fluoroscopy cannot identify the site of origin of an arrhythmia and thus cannot be used to specifically direct a catheter to that site.
Electro-anatomic mapping systems (e.g., Carto, Biosense, Marlton, N.J.) provide an electro-anatomical map of the heart. This method of nonfluoroscopic catheter mapping is based on an activation sequence to track and localize the tip of the mapping catheter by magnetic localization in conjunction with electrical activity recorded by the catheter. This approach has been used in ventricular tachyardia [Nademanee, 1998; Stevenson, 1998], atrial flutter [Shah, 1997; Nakagawa, 1998], and atrial tachycardia ablation [Natale, 1998; Kottkamp, 1997]. The ability to localize in space the tip of the catheter while simultaneously measuring the electrical activity may facilitate the mapping process. However, this technique fundamentally has the limitation that it involves sequentially sampling endocardial sites. The mapping process is prolonged while the patients must be maintained in VT. Also, the localization is limited to the endocardial surface and thus sites of origin within the myocardium cannot be accurately localized.
The basket catheter technique employs a non-contact 64-electrode basket catheter (Endocardial Solutions Inc., St. Paul, Minn.) placed inside the heart to electrically map the heart. In the first part of this procedure high frequency current pulses are applied to a standard catheter used in an ablation procedure. The tip of this catheter is dragged over the endocardial surface, and a basket catheter is used to locate the tip of the ablation catheter and thus to trace and reconstruct the endocardial surface of the ventricular chamber. Then the chamber geometry, the known locations of the basket catheter, and the non-contact potential at each electrode on the basket catheter are combined in solving Laplace""s equation, and electrograms on the endocardial surface are computed. This technique has been used in mapping atrial and ventricular arrhythmias [Schilling, 1998; Gornick, 1999]. One of the drawbacks of this methodology is that the ventricular geometry is not fixed but varies during the cardiac cycle. In addition, the relative movement between the constantly contracting heart and the electrodes affects the mapping. While the inter-electrode distances on each sidearm of the basket catheter are fixed, the distances between the actual recording sites on the endocardium decrease during systole. This leads to relative movement between the recording electrode and the tissue, significantly limiting the accuracy of the mapping method. Also, the localization is limited to the endocardial surface, and thus sites of origin within the myocardium cannot be accurately localized.
What is needed is a means of efficiently directing the tip of a catheter to a site of origin of an arrhythmia in the heart (whether on the endocardial surface or within the myocardium itself), without the need to introduce additional passive electrodes into the heart, so that energy can be delivered through the catheter to ablate the site of origin. It would be advantageous to be able to accomplish this task without having to maintain the arrhythmia while advancing the catheter to the site of origin, so that the patient does not suffer the ill effects of the arrhythmia for a prolonged period. This consideration is particularly important in the case of rapid arrhythmias that compromise hemodynamic function.
The present invention provides methods and apparatus for localizing an electrical source within the body. The invention further provides methods and apparatus for delivering ablative electrical energy in the vicinity of an electrical source within the body. The electrical source may be located anywhere within the body. For example, the electrical source may be within the heart and may be the site of origin of a cardiac arrhythmia. The electrical source may be a focus of electrical activity within the brain, such as a site involved in triggering an epileptic seizure, or may be located in other neurological tissue.
Cardiac arrhythmias are frequently treated by delivering electrical energy to the site of origin of the arrhythmia in an effort to ablate the site. To effectively perform this procedure, accurate localization of both the site of origin of the arrhythmia and the energy delivery device (e.g., the tip of a catheter) is necessary. As used herein, the term xe2x80x9clocalizationxe2x80x9d refers to determining either an absolute or a relative location. The present invention provides techniques for accurately performing such localization. The minimally invasive and fast aspects of certain embodiments of the invention, as disclosed herein, are particularly important.
In preferred embodiments the methods of the present invention involve placing passive electrodes on the body surface, placing active electrode(s) in and/or on the body, acquiring from the passive electrodes signals emanating from the electrical source, processing the signals emanating from the electrical source to determine the relative location of the electrical source, delivering electrical energy to the active electrode(s), acquiring from the passive electrodes the signals emanating from the active electrode(s), processing the signals emanating from the active electrode(s) to determine the relative location of the active electrode(s), and positioning the active electrode(s) to localize the electrical source. In another embodiment at least one of the passive electrodes is placed within the body, for example within the heart. The positioning step of the present invention may involve approximating the relative locations of the active electrode(s) and the electrical source. In preferred embodiments of the method the energy delivering step, the second acquiring step, the second processing step and the positioning step are performed iteratively.
In a preferred embodiment the first processing step is used to determine the relative location of the electrical source at a multiplicity of time epochs during the cardiac cycle, and the positioning step localizes the electrical source at one of the time epochs. At least one criterion may be used to choose the time epoch. In a particularly preferred embodiment at least one of the processing steps involves fitting the acquired signals to a moving dipole model. In a particularly preferred embodiment of the invention the second processing step includes determining the relative location of a moving dipole that is approximately parallel to the moving dipole fitted in the first processing step to signals emanating from the electrical source. In one embodiment, such determination is made using a multiplicity of active electrodes.
Another preferred embodiment of the invention involves delivering ablative energy in the vicinity of the location of an electrical source within the body by delivering ablative energy in the vicinity of the location of the active electrode(s). The active electrode(s) may be located on a catheter, and the ablative energy may be delivered through the catheter. In a preferred embodiment the ablative energy is radio frequency energy.
The methods of the present invention may further include displaying various parameters. Among the parameters of interest are the relative location of the electrical source and measures of the size, strength, and/or uncertainty in the relative location of the electrical source.
Other features and advantages of the invention will become apparent from the following description, including the drawing, and from the claims.